Optical amplifier having a substantially flat gain spectrum

Abstract

An optical amplifier is provided in which optical channels, each at a respective wavelength, make two passes through a segment of erbium-doped optical fiber. After the first pass, certain optical wavelengths lying in the high gain spectrum (“the high gain wavelengths”) of the erbium-doped optical fiber are amplified more than other optical wavelengths lying in the low gain spectrum (“the low gain wavelengths”). The optical channels are then reflected with a reflective element back to the segment of erbium-doped optical fiber for the second pass. The reflective element selectively attenuates the high gain wavelengths to compensate for the excessive gain of the erbium-doped optical fiber at these wavelengths. As a result, after the second pass, the optical power at the high and low gain wavelengths is substantially the same and gain flattening is achieved. In an alternative embodiment, the low gain wavelengths are selectively amplified by the reflective element and supplied to the erbium-doped optical fiber at a higher power level than the high gain wavelengths. This additional optical power offsets the low amplification at the low gain wavelengths, such that the erbium-doped optical fiber outputs the high and low gain wavelengths at substantially the same optical power levels.

What is claimed is:  
     
       1. An optical amplifier, comprising: 
       a first segment of optically amplifying fiber configured to receive a plurality of optical signals at a first end portion, each of said plurality of optical signals being at a respective one of a plurality of wavelengths, said first segment of doped optical fiber having a second end portion;  
       an optical circulator having first, second and third ports, said first port of said optical circulator being coupled to said second end portion of said first segment of doped optical fiber, said optical circulator being coupled to circulate said plurality of optical signal from said first port to said second port;  
       a second segment of optically amplifying fiber having a first end portion coupled to said second port of said optical circulator and a second end portion; and  
       an optically reflective element coupled to said second end portion of said second segment of optically amplifying fiber, said optically reflective element having a power spectrum that substantially compensates a power spectrum associated with said plurality of optical signals after a single pass through said first segment of optically amplifying fiber and two passes through said second segment of optically amplifying fiber.  
     
     
       2. An optical amplifier in accordance with claim  1 , wherein said optically reflective element comprises an in-fiber Bragg grating. 
     
     
       3. An optical amplifier in accordance with claim  1 , wherein said optical circulator is a first optical circulator, said optical amplifier further comprising: 
       a second optical circulator having a first port coupled to said second port of said first optical circulator, a second port and a third port; and  
       a transmission mode dielectric filter having an input coupled to said second port of said second optical circulator and an output coupled to said third port of said second optical circulator.  
     
     
       4. An optical amplifier in accordance with claim  1 , wherein said optically reflective element comprises: 
       a first plurality of in-fiber Bragg gratings, each configured to reflect a respective one of a first group of said plurality of optical signals;  
       an optical attenuator having an input coupled to said first plurality of in-fiber Bragg gratings and an output; and  
       a second plurality of in-fiber Bragg gratings, each configured to reflect a respective one of a second group of said plurality of optical signals, said second plurality of in-fiber Bragg gratings being coupled to said output of said optical attenuator.  
     
     
       5. An optical amplifier comprising: 
       a first segment of optically amplifying fiber configured to receive a plurality of optical signals at a first end portion, each of said plurality of optical signals being at a respective one of a plurality of wavelengths, said first segment of doped optical fiber having a second end portion;  
       an optical circulator having first, second and third ports, said first port of said optical circulator being coupled to said second end portion of said first segment of doped optical fiber, said optical circulator being coupled to circulate said plurality of optical signal from said first port to said second port;  
       a second segment of optically amplifying fiber having a first end portion coupled to said second port of said optical circulator and a second end portion;  
       a first plurality of in-fiber Bragg gratings, each configured to reflect a respective one of a first group of said plurality of optical signals;  
       a third segment of optically amplifying fiber coupled to said first plurality of in-fiber Bragg gratings; and  
       a second plurality of in-fiber Bragg gratings, each configured to reflect a respective one of a second group of said plurality of optical signals, said second plurality of in-fiber Bragg gratings being coupled to said third segment of optically amplifying fiber.  
     
     
       6. A method of amplifying a plurality of optical signals, wherein each of said plurality of optical signals being at a respective one of a plurality of wavelengths, said method comprising the steps of: 
       non-uniformly amplifying said plurality of optical signals with a first optically amplifying fiber;  
       supplying said amplified plurality of optical signals to a second optically amplifying fiber;  
       non-uniformly amplifying said plurality of optical signals with said second optically amplifying fiber;  
       reflecting said amplified plurality of optical signals from said second optically amplifying fiber back to said second optically amplifying fiber;  
       selectively attenuating said reflected optical signals with a power spectrum that substantially compensates a power spectrum associated with said plurality of optical signals after a single pass through said first segment of optically amplifying fiber and two passes through said second segment of optically amplifying fiber; and  
       non-uniformly amplifying said reflected and attenuated plurality of optical signals with said second optically amplifying fiber such that said plurality of optical signals having substantially the same power level.